1. Field of the Invention
The present invention relates to a vehicle brake hydraulic pressure control apparatus which utilizes normally open linear solenoid valves as inlet valves.
2. Description of Related Art
In general, there is known a vehicle brake hydraulic pressure control apparatus which is disposed between a master cylinder which generates a hydraulic pressure according to a pedal effort applied by the driver and wheel brakes which apply brake forces to road wheels for controlling brake forces generated at the wheel brakes.
A vehicle brake hydraulic pressure control apparatus like this includes mainly normally open inlet valves for allowing transmission of the brake hydraulic pressure from the master cylinder side to the wheel brakes, normally closed outlet valves for allowing hydraulic pressures acting on the wheel brakes to escape therefrom, reservoirs for reserving temporarily the brake hydraulic pressures which are allowed to escape from the wheel cylinders by the outlet valves being opened and pumps for returning brake fluid within the reservoirs towards the inlet valves.
In addition, as the vehicle brake hydraulic pressure control apparatus that has been described above, there is conventionally known a vehicle brake hydraulic pressure control apparatus which adopts normally open linear solenoid valves (linear solenoid valves) as inlet valves (refer to Japanese Patent Unexamined Publication JP-A-2003-19952). Specifically, the normally open linear solenoid valve includes a valve seat, a valve body which is normally biased in a valve opening direction from the valve seat by a spring, a core for drawing the valve body towards the valve seat by virtue of magnetic force and an exciting coil for exciting the core.
Additionally, when the inlet valve is made up of the normally open linear solenoid valve like above, a valve closing force (a force with which the valve body is pressed against the valve seat) can be changed arbitrarily by controlling an energization amount supplied to the exciting coil. Because of this, since the valve opening amount (a gap between the valve seat and the valve body) can be controlled by controlling the energization amount appropriately by taking into consideration a differential pressure between an upstream pressure and a downstream pressure of the inlet valve, hydraulic pressure within the wheel brake can be increased linearly (proportionally).
In the related art, however, there may be caused a situation in which disturbance such as discharge pulsation from the pump is generated on an upstream side of the inlet valve when increasing the pressure within the wheel brake. As this occurs, even though the inlet valve is attempted to be opened by a target valve opening amount, by taking advantage of the valve body of the inlet valve being affected by the pulsation, the balance between the differential pressure between the upstream pressure and the downstream pressure of the inlet valve and the energization amount is lost and the valve body produces self-oscillation. Due to this, there is a fear that so-called hunting, in which the valve body does not converge on a target position (a balanced position), is caused. When the hunting like this occurs, there has been a possibility that piping connected to the vehicle brake hydraulic pressure control apparatus resonates due to the pulsation of hydraulic pressure as a result of the hunting so as to produce noise.